1. Field of the Invention
The present invention relates to a technique for reliably shifting to a stable lighting state after a discharge lamp is turned on in a discharge lamp lighting circuit suited for compact design and supporting high-frequencies.
2. Description of the Related Art
There is known a lighting circuit for a discharge lamp such as a metal halide lamp having a DC power supply circuit designed as a DC-to-DC converter, a DC-to-AC converter circuit, and a starter circuit. For example, an input DC voltage from a battery is converted to a desired voltage in the DC power supply circuit and then converted to an AC output in the DC-to-AC converter circuit downstream of the DC power supply circuit, and the output is overlaid with a starting signal. The resulting signal is supplied to a discharge lamp (For example, refer to JP-A-7-142182).
In the process of lighting control of a discharge lamp, an open-circuit voltage (hereinafter referred to as OCV) before the discharge lamp is lit (when the discharge lamp is turned off) is controlled to apply a start signal to the discharge lamp thereby lighting the discharge lamp and lowering a transient input voltage to place the discharge lamp in the steady lighting state.
The DC power supply circuit comprises for example a switching regulator that uses a transformer. The DC-to-AC converter circuit comprises, for example, a full bridge type design using multiple pairs of switching elements.
JP-A-7-142182 is referred to as a related art.
A related art lighting circuit requires a transformer used in a DC power supply circuit and a transformer that constitutes a starting circuit. Further, the larger the number of switching elements used in a DC-to-AC converter circuit becomes, the more problems arise with the circuit scale and the system cost. For example, in case a discharge lamp is used as a light source for an automobile lamp, it is necessary to arrange a lighting circuit in a limited space (such as a case where a lighting circuit unit is housed in a lighting fixture).
In a configuration where voltage conversion is performed at two stages (DC voltage conversion and DC-TO-AC voltage conversion), the circuit scale could be increased, which compromises a compact design. In order to offset this drawback, a configuration is possible where an output boosted by single-stage voltage conversion in a DC-to-AC converter circuit is supplied to a discharge lamp. For example, a configuration is possible where a single transformer and a resonance circuit are used to boost a resonance voltage and the resulting power is supplied to a discharge lamp. What counts in such a case is how the discharge lamp is reliably and quickly placed in a stable lighting state after it is started. This need is mandatory for safety in nighttime operation in an application of a light source for an automobile lamp. In particular, in a case where a discharge lamp is to be lit when it is cold (so-called “cold start”), an excessive input power exceeding a rated power is supplied to the discharge lamp. It is necessary to provide countermeasures against a possible rise in the probability of a blown lamp taking place in case discharge is interrupted when the discharge lamp is no longer lit during transient power control.